.\" Copyright (C) 2001 by Martin C. Shepherd
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.TH pca_lookup_file 3
.SH NAME
pca_lookup_file, del_PathCache, del_PcaPathConf, new_PathCache, new_PcaPathConf, pca_last_error, pca_path_completions, pca_scan_path, pca_set_check_fn, ppc_file_start, ppc_literal_escapes \- lookup a file in a list of directories
.SH SYNOPSIS
.nf
#include <libtecla.h>

PathCache *new_PathCache(void);

PathCache *del_PathCache(PathCache *pc);

int pca_scan_path(PathCache *pc, const char *path);

void pca_set_check_fn(PathCache *pc, CplCheckFn *check_fn,
                      void *data);

char *pca_lookup_file(PathCache *pc, const char *name,
                      int name_len, int literal);

const char *pca_last_error(PathCache *pc);

CPL_MATCH_FN(pca_path_completions);

.fi

.SH DESCRIPTION

The \f3PathCache\f1 object is part of the tecla library (see the
libtecla(3) man page).
.sp
\f3PathCache\f1 objects allow an application to search for files in
any colon separated list of directories, such as the unix execution
PATH environment variable. Files in absolute directories are cached in
a \f3PathCache\f1 object, whereas relative directories are scanned as
needed. Using a \f3PathCache\f1 object, you can look up the full
pathname of a simple filename, or you can obtain a list of the
possible completions of a given filename prefix. By default all files
in the list of directories are targets for lookup and completion, but
a versatile mechanism is provided for only selecting specific types of
files. The obvious application of this facility is to provide
Tab-completion and lookup of executable commands in the unix PATH, so
an optional callback which rejects all but executable files, is
provided.
.sp
.SH AN EXAMPLE

Under UNIX, the following example program looks up and displays the
full pathnames of each of the command names on the command line.
.sp
.nf
  #include <stdio.h>
  #include <stdlib.h>
  #include <libtecla.h>

  int main(int argc, char *argv[])
  {
    int i;
  /*
   * Create a cache for executable files.
   */
    PathCache *pc = new_PathCache();
    if(!pc)
      exit(1);
  /*
   * Scan the user's PATH for executables.
   */
    if(pca_scan_path(pc, getenv("PATH"))) {
      fprintf(stderr, "%s\\n", pca_last_error(pc));
      exit(1);
    }
  /*
   * Arrange to only report executable files.
   */
   pca_set_check_fn(pc, cpl_check_exe, NULL);
  /*
   * Lookup and display the full pathname of each of the
   * commands listed on the command line.
   */
    for(i=1; i<argc; i++) {
      char *cmd = pca_lookup_file(pc, argv[i], -1, 0);
      printf("The full pathname of '%s' is %s\\n", argv[i],
             cmd ? cmd : "unknown");
    }
    pc = del_PathCache(pc);  /* Clean up */
    return 0;
  }
.fi
.sp
The following is an example of what this does on my laptop under
linux:
.sp
.nf
  $ ./example less more blob
  The full pathname of 'less' is /usr/bin/less
  The full pathname of 'more' is /bin/more
  The full pathname of 'blob' is unknown
  $ 
.fi
.sp
.SH FUNCTION DESCRIPTIONS

In order to use the facilities of this module, you must first allocate
a \f3PathCache\f1 object by calling the \f3new_PathCache()\f1
constructor function.
.sp
.nf
  PathCache *new_PathCache(void)
.fi
.sp
This function creates the resources needed to cache and lookup files
in a list of directories. It returns \f3NULL\f1 on error.
.sp
.SH POPULATING THE CACHE
Once you have created a cache, it needs to be populated with files.
To do this, call the \f3pca_scan_path()\f1 function.
.sp
.nf
  int pca_scan_path(PathCache *pc, const char *path);
.fi
.sp
Whenever this function is called, it discards the current contents of
the cache, then scans the list of directories specified in its
\f3path\f1 argument for files. The \f3path\f1 argument must be a
string containing a colon-separated list of directories, such as
\f3"/usr/bin:/home/mcs/bin:."\f1. This can include directories
specified by absolute pathnames such as \f3"/usr/bin"\f1, as well as
sub-directories specified by relative pathnames such as \f3"."\f1 or
\f3"bin"\f1. Files in the absolute directories are immediately cached
in the specified \f3PathCache\f1 object, whereas sub-directories,
whose identities obviously change whenever the current working
directory is changed, are marked to be scanned on the fly whenever a
file is looked up.
.sp
On success this function return \f30\f1. On error it returns \f31\f1,
and a description of the error can be obtained by calling
\f3pca_last_error(pc)\f1.
.sp
.SH LOOKING UP FILES

Once the cache has been populated with files, you can look up the full
pathname of a file, simply by specifying its filename to
\f3pca_lookup_file()\f1.
.sp
.nf
  char *pca_lookup_file(PathCache *pc, const char *name,
                        int name_len, int literal);
.fi
.sp
To make it possible to pass this function a filename which is actually
part of a longer string, the \f3name_len\f1 argument can be used to
specify the length of the filename at the start of the \f3name[]\f1
argument. If you pass \f3-1\f1 for this length, the length of the
string will be determined with \f3strlen()\f1. If the \f3name[]\f1
string might contain backslashes that escape the special meanings of
spaces and tabs within the filename, give the \f3literal\f1 argument,
the value \f30\f1. Otherwise, if backslashes should be treated as
normal characters, pass \f31\f1 for the value of the \f3literal\f1
argument.

.SH FILENAME COMPLETION

Looking up the potential completions of a filename-prefix in the
filename cache, is achieved by passing the provided
\f3pca_path_completions()\f1 callback function to the
\f3cpl_complete_word()\f1 function (see the \f3cpl_complete_word(3)\f1
man page).
.sp
.nf
  CPL_MATCH_FN(pca_path_completions);
.fi
.sp
This callback requires that its \f3data\f1 argument be a pointer to a
\f3PcaPathConf\f1 object. Configuration objects of this type are
allocated by calling \f3new_PcaPathConf()\f1.
.sp
.nf
  PcaPathConf *new_PcaPathConf(PathCache *pc);
.fi
.sp
This function returns an object initialized with default configuration
parameters, which determine how the \f3cpl_path_completions()\f1
callback function behaves. The functions which allow you to
individually change these parameters are discussed below.
.sp
By default, the \f3pca_path_completions()\f1 callback function
searches backwards for the start of the filename being completed,
looking for the first un-escaped space or the start of the input
line. If you wish to specify a different location, call
\f3ppc_file_start()\f1 with the index at which the filename starts in
the input line. Passing \f3start_index=-1\f1 re-enables the default
behavior.
.sp
.nf
  void ppc_file_start(PcaPathConf *ppc, int start_index);
.fi
.sp
By default, when \f3pca_path_completions()\f1 looks at a filename in
the input line, each lone backslash in the input line is interpreted
as being a special character which removes any special significance of
the character which follows it, such as a space which should be taken
as part of the filename rather than delimiting the start of the
filename. These backslashes are thus ignored while looking for
completions, and subsequently added before spaces, tabs and literal
backslashes in the list of completions. To have unescaped backslashes
treated as normal characters, call \f3ppc_literal_escapes()\f1 with a
non-zero value in its \f3literal\f1 argument.
.sp
.nf
  void ppc_literal_escapes(PcaPathConf *ppc, int literal);
.fi
.sp
When you have finished with a \f3PcaPathConf\f1 variable, you can pass
it to the \f3del_PcaPathConf()\f1 destructor function to reclaim its
memory.
.sp
.nf
  PcaPathConf *del_PcaPathConf(PcaPathConf *ppc);
.fi
.sp

.SH BEING SELECTIVE
If you are only interested in certain types or files, such as, for
example, executable files, or files whose names end in a particular
suffix, you can arrange for the file completion and lookup functions
to be selective in the filenames that they return.  This is done by
registering a callback function with your \f3PathCache\f1
object. Thereafter, whenever a filename is found which either matches
a filename being looked up, or matches a prefix which is being
completed, your callback function will be called with the full
pathname of the file, plus any application-specific data that you
provide, and if the callback returns \f31\f1 the filename will be
reported as a match, and if it returns \f30\f1, it will be ignored.
Suitable callback functions and their prototypes should be declared
with the following macro. The \f3CplCheckFn\f1 \f3typedef\f1 is also
provided in case you wish to declare pointers to such functions.
.sp
.nf
  #define CPL_CHECK_FN(fn) int (fn)(void *data, \\
                                    const char *pathname)
  typedef CPL_CHECK_FN(CplCheckFn);
.fi
.sp
Registering one of these functions involves calling the
\f3pca_set_check_fn()\f1 function. In addition to the callback
function, passed via the \f3check_fn\f1 argument, you can pass a
pointer to anything via the \f3data\f1 argument. This pointer will be
passed on to your callback function, via its own \f3data\f1 argument,
whenever it is called, so this provides a way to pass appplication
specific data to your callback.
.sp
.nf
  void pca_set_check_fn(PathCache *pc, CplCheckFn *check_fn,
                        void *data);
.fi
.sp
Note that these callbacks are passed the full pathname of each
matching file, so the decision about whether a file is of interest can
be based on any property of the file, not just its filename. As an
example, the provided \f3cpl_check_exe()\f1 callback function looks at
the executable permissions of the file and the permissions of its
parent directories, and only returns \f31\f1 if the user has execute
permission to the file. This callback function can thus be used to
lookup or complete command names found in the directories listed in
the user's \f3PATH\f1 environment variable. The example program given
earlier in this man page provides a demonstration of this.
.sp
Beware that if somebody tries to complete an empty string, your
callback will get called once for every file in the cache, which could
number in the thousands. If your callback does anything time
consuming, this could result in an unacceptable delay for the user, so
callbacks should be kept short.
.sp
To improve performance, whenever one of these callbacks is called, the
choice that it makes is cached, and the next time the corresponding
file is looked up, instead of calling the callback again, the cached
record of whether it was accepted or rejected is used. Thus if
somebody tries to complete an empty string, and hits tab a second time
when nothing appears to happen, there will only be one long delay,
since the second pass will operate entirely from the cached
dispositions of the files. These cached dipositions are discarded
whenever \f3pca_scan_path()\f1 is called, and whenever
\f3pca_set_check_fn()\f1 is called with changed callback function or
data arguments.

.SH ERROR HANDLING

If \f3pca_scan_path()\f1 reports that an error occurred by returning
\f31\f1, you can obtain a terse description of the error by calling
\f3pca_last_error(pc)\f1. This returns an internal string containing
an error message.
.sp
.nf
  const char *pca_last_error(PathCache *pc);
.fi
.sp

.SH CLEANING UP

Once you have finished using a \f3PathCache\f1 object, you can reclaim
its resources by passing it to the \f3del_PathCache()\f1 destructor
function. This takes a pointer to one of these objects, and always
returns \f3NULL\f1.
.sp
.nf
  PathCache *del_PathCache(PathCache *pc);
.fi
.sp
.SH THREAD SAFETY

In multi-threaded programs, you should use the \f3libtecla_r.a\f1
version of the library. This uses POSIX reentrant functions where
available (hence the \f3_r\f1 suffix), and disables features that rely
on non-reentrant system functions. In the case of this module, the
only disabled feature is username completion in \f3~username/\f1
expressions, in \f3cpl_path_completions()\f1.

Using the \f3libtecla_r.a\f1 version of the library, it is safe to use
the facilities of this module in multiple threads, provided that each
thread uses a separately allocated \f3PathCache\f1 object. In other
words, if two threads want to do path searching, they should each call
\f3new_PathCache()\f1 to allocate their own caches.

.SH FILES
.nf
libtecla.a    -    The tecla library
libtecla.h    -    The tecla header file.
.fi

.SH SEE ALSO
libtecla(3), gl_get_line(3), ef_expand_file(3), cpl_complete_word(3)
  
.SH AUTHOR
Martin Shepherd  (mcs@astro.caltech.edu)
